Hybrid vehicles are driven by multiple powerplants including, but not limited to an internal combustion engine and an electric machine. The electric machine functions as a motor/generator. In a generator mode, the electric machine is driven by the engine to generate electrical energy used to power electrical loads or charge batteries. In a motor mode, the electric machine supplements the engine, providing drive torque to drive the vehicle drivetrain.
In some instances, the fuel is cut-off to the engine during deceleration (referred to as deceleration fuel cut-off (DFCO)) or electric motoring of the engine. During these events, the engine is back driven, which induces reciprocal movement of the cylinders within the engine. As a result of the back driving of the engine, the manifold absolute pressure (MAP) of an intake manifold of the engine is significantly reduced, resulting in reduced efficiency and other losses. More specifically, reduced MAP results in low pressure in the intake ports and in the combustion chamber (i.e., within the cylinder in the space above the piston).
If the pressure is too low, the oil can be drawn out of the engine crankcase through the piston rings, the valve guides disposed within the inlet ports, as well as through other paths. The piston rings can also flutter or vibrate depending on dynamic pressure differences between the crank case and the combustion chamber, which can be detrimental to the engine and may be perceptible by vehicle occupants.